1. Field of the Invention
The present invention relates to a distance measuring device, a distance measuring method and a distance measuring program. More specifically, the present invention relates to a distance measuring device, a distance measuring method and a distance measuring program for measuring distance to an object of measurement using an electromagnetic wave emitted to the object.
2. Description of the Background Art
Methods of measuring distance to an object of detection utilizing a microwave generally used at present is roughly classified into FMCW (Frequency Modulated Continuous Wave) based method and pulse-radar based method.
In the FMCW based method, frequency-swept continuous waves are sent, and from frequency difference between the emitted signal and a reflected signal, the distance to the object of detection is found (see, for example, Japanese Patent Laying-Open No. 07-159522).
In pulse radar, the time from sending a pulse signal until the pulse signal is reflected by the object of detection and returns is measured, and the distance to the object of detection is found from the measured time (see International Publication No. WO94/24579).
Though the two methods both have high measurement accuracy, the methods respectively have the following problems.
First, in the FMCW method, the measurement accuracy is determined by the sweep width of emission frequency, as represented by the equation: measurement accuracy=speed of light/(2×width of frequency sweep), and therefore, in order to attain high accuracy, it is necessary to use wide bandwidth. In the frequency band of 24.15 GHz specified by the Radio Law as the band for sensors for detecting moving object, which is normally used by the distance measuring device, available bandwidth is limited to 0.1 GHz from effective frequency of 24.1 to 24.2 GHz, because of regulations on specified low power radio station. Therefore, bandwidth is insufficient for outdoor-use of an FMCW microwave type level meter, so that the measurement accuracy is limited and measurement of short distance is difficult.
Second, in the pulse-radar method, in order to generate very short electric pulses by an emitter, wide radio wave bandwidth is necessary when the components are considered. By way of example, the bandwidth necessary for generating an impulse of 2n second is 2 GHz. Therefore, in this case also, outdoor use is limited because of the restriction of bandwidth defined by the Radio Law, and measurement of short distance is difficult, as shorter electric pulses cannot be used.
In order to solve these problems, it is necessary to satisfy the radio wave bandwidth and emission gain, and to maintain high measurement accuracy regardless of the measurement distance, particularly at a short distance.
The two methods of measurement use wide bandwidth, and therefore, these cannot be used as the specified low power radio station as classified by the Radio Law. These methods, however, can be used as an extremely low power radio station, with the output power kept low. When the output power of emission signal is made lower, however, the power of the reflected signal also comes to be very low. This leads to a problem that measurement of long distance comes to be highly susceptible to the influence of noise.
Recently, a distance measuring device having high measurement accuracy even for a short distance is proposed (see, for example, Japanese Patent Laying-Open No. 2002-357656).
FIG. 10 is a schematic block diagram representing an arrangement of distance measuring device proposed in Japanese Patent Laying-Open No. 2002-357656.
Referring to FIG. 10, the distance measuring device includes: a sending source 60 outputting a signal of a prescribed frequency; a transmission unit 70 emitting an electromagnetic wave of the same frequency as the output signal of sending source 60; a detecting unit 80 detecting an amplitude of a standing wave S formed by an interference between the electromagnetic wave (hereinafter also referred to as a traveling wave D) emitted from transmission unit 70 and a reflected wave R reflected by objects of measurement M1 to Mn (n is a natural number); and a signal processing unit 90 calculating the distance to the object of measurement Mk (k is a natural number not larger than n) from the detection signal of detecting unit 80.
Sending source 60 includes a sending unit 62 and a frequency control unit 64. Sending unit 62 outputs a signal of constant frequency f controlled by frequency control unit 64 to transmission unit 70. Frequency control unit 64 outputs information related to the frequency f sent to sending unit 62 also to signal processing unit 90.
The principle of measurement in the distance measuring device shown in FIG. 10 will be briefly described.
First, as shown in FIG. 10, the traveling wave D emitted from transmission unit 70 interferes with the reflected wave R reflected by the object of measurement Mk, so that a standing wave S is formed in the propagation medium between transmission unit 70 and the object of measurement Mk.
At this time, a reception power signal p(f, x) obtained through monitoring of the standing wave S at a detecting unit 80 provided on a point of monitoring xs on the x-axis is a sinusoidal wave function (cos function) of the frequency f of traveling wave D. Particularly when there are reflections from a plurality of objects of measurement, the signal would be a composition of a plurality of sinusoidal waves having mutually different periods corresponding to respective objects of measurement. The period of each sinusoidal wave is in inverse proportion to the distance from the point of monitoring to the object of measurement Mk. The distance measuring device shown in FIG. 10 measures the distance to the object Mk utilizing this characteristic.
Specifically, the standing wave S is generated by additive composition of traveling wave D emitted from transmission unit 70 and reflected wave R reflected from the object Mk, of which power signal p(f, x) is given by:
                              p          ⁡                      (                          f              ,              x                        )                          ≈                              A            2                    +                      2            ⁢                                          ∑                                  k                  =                  1                                n                            ⁢                                                A                  2                                ⁢                                  γ                  k                                ⁢                                  cos                  ⁡                                      (                                                                                                                        4                            ⁢                                                          π                              ⁡                                                              (                                                                                                      d                                    k                                                                    -                                  x                                                                )                                                                                                              c                                                ⁢                        f                                            +                                              ϕ                        k                                                              )                                                                                                          Equation        ⁢                                  ⁢                  (          1          )                    
where c represents speed of light, f represents transmission frequency, A represents amplitude level of traveling wave D, and dk represents distance to the object of measurement Mk. Further, γk represents magnitude of reflection coefficient of the object of measurement Mk including propagation loss, and φk represents an amount of phase shift in reflection.
FIG. 11 is a diagram of waveform of the reception power signal p(f, 0) monitored at the position of x=xs=0, when the object Mk is positioned at a distance dk.
It can be seen from FIG. 11 that the reception power signal p(f, 0) is periodical with respect to the transmission frequency f. Further, it can be seen that the period is c/2d, and is in inverse proportion to the distance d to the object of measurement.
Therefore, by Fourier transform of the reception power signal p(f, 0) to extract period information, the distance d to the object of measurement can be found. Here, a profile P(x) obtained by applying Fourier transform on the reception power signal p(f, 0) of Equation (1) is expressed as:
                                                                                          P                  ⁡                                      (                    x                    )                                                  =                                ⁢                                                                            ∫                                                                        f                          0                                                -                                                                              f                            W                                                    2                                                                                                                      f                          0                                                +                                                                              f                            W                                                    2                                                                                      ⁢                                                                  p                        ⁢                                                  (                                                      f                            ,                            0                                                    )                                                                    ⁢                                              ⅇ                                                                              -                            j                                                    ⁢                                                                                    4                              ⁢                              π                              ⁢                                                                                                                          ⁢                              x                                                        c                                                    ⁢                          f                                                                    ⁢                                              ⅆ                        f                                                                              =                                                                                                                        ⁢                                                      f                    W                                    ⁢                                      A                    2                                    ⁢                                      ⅇ                                                                  -                        j                                            ⁢                                                                        4                          ⁢                          π                          ⁢                                                                                                          ⁢                          x                                                c                                            ⁢                                              f                        0                                                                              ⁢                                      {                                                                                            S                          a                                                ⁡                                                  (                                                                                                                    2                                ⁢                                π                                ⁢                                                                                                                                  ⁢                                                                  f                                  W                                                                                            c                                                        ⁢                            x                                                    )                                                                    +                                                                                                                                                              ⁢                                                                            ∑                                              k                        =                        1                                            n                                        ⁢                                                                  γ                        k                                            ⁢                                              ⅇ                                                  -                                                      j                            ⁡                                                          (                                                                                                                                                                          4                                      ⁢                                      π                                      ⁢                                                                              ⅆ                                        k                                                                                                              c                                                                    ⁢                                                                      f                                    0                                                                                                  +                                                                  ϕ                                  k                                                                                            )                                                                                                                          ⁢                                                                        S                          a                                                ⁡                                                  (                                                                                                                    2                                ⁢                                π                                ⁢                                                                                                                                  ⁢                                                                  f                                  W                                                                                            c                                                        ⁢                                                          (                                                              x                                -                                                                  d                                  k                                                                                            )                                                                                )                                                                                                      +                                                                                                                        ⁢                                                      ∑                                          k                      =                      1                                        n                                    ⁢                                                            γ                      k                                        ⁢                                          ⅇ                                              -                                                  j                          ⁡                                                      (                                                                                                                                                                4                                    ⁢                                    π                                    ⁢                                                                          ⅆ                                      k                                                                                                        c                                                                ⁢                                                                  f                                  0                                                                                            +                                                              ϕ                                k                                                                                      )                                                                                                                ⁢                                                                  S                        a                                            ⁡                                              (                                                                                                            2                              ⁢                              π                              ⁢                                                                                                                          ⁢                                                              f                                W                                                                                      c                                                    ⁢                                                      (                                                          x                              +                                                              d                                k                                                                                      )                                                                          )                                                                                            }                                                    ⁢                                  ⁢        where                            Equation        ⁢                                  ⁢                  (          2          )                                                              S            a                    ⁡                      (            z            )                          =                              sin            ⁡                          (              z              )                                z                                    Equation        ⁢                                  ⁢                  (          3          )                    
Here, f0 represents intermediate frequency of the transmission frequency band, and fw represents bandwidth of the transmission frequency.
Namely, in the distance measuring device of FIG. 10, the distance dk to the object of measurement Mk depends solely on the vibration period of reception power signal p(f, 0) with respect to the transmission frequency f of the traveling wave D, and not influenced by the time from the emission of electromagnetic wave from transmission unit 70 until return to detecting unit 80. Therefore, it is possible to measure a short distance with higher accuracy than the conventional FMCW method and pulse radar method.
Here, in the conventional distance measuring device shown in FIG. 10, the reception power signal p(f, 0) of the standing wave S is subjected to Fourier transform in accordance with Equation (2), and therefore, accurate periodic information can be extracted only when the reception power signal p(f, x) has periodicity of at least one period in the bandwidth fw of transmission frequency.
FIG. 12 represents magnitude |P(x)| of the profile of reception power signal when the distance dk of the object of measurement Mk is changed in the range of 0 m≦dk≦5 m under the conditions that f0=24.0375 GHz, fw=75 MHz, γk=0.1 and φk=π, found through calculations in accordance with Equations (1) and (2). Here, p(f, 0)−A2, with the level A2 of traveling wave subtracted, is subjected to Fourier transform, and therefore, the first term of Equation (1) is removed.
Referring to FIG. 12, the profile magnitude |P(x)| comes to have a waveform that has local maximums both in a region where x is positive and in a region where x is negative, corresponding to the components of the second and third terms of Equation (2). In the conventional distance measuring device, for the object of measurement Mk, x is always in the positive region, and therefore, the local maximum in one region (x>0) of the waveform is extracted and the value x that corresponds to the local maximum is determined to be the position of the object Mk.
When the distance d is small, however, the peak of profile magnitude |P(x)| does not indicate the accurate position of the object Mk, as shown in FIG. 12. The reason for this is as follows. As the distance d becomes smaller, two local maximums come to interfere with each other, making the waveform irregular. In the example shown in FIG. 12, it can be seen that accurate measurement is possible when the distance d is 2 m or longer, while a correct measurement cannot be obtained when the distance d becomes shorter than 2 m.
Specifically, the conventional distance measuring device has a problem of increased measurement error at shorter distance, which derives from the influence of the local maximum appearing in the region where x is negative on the local maximum appearing in the region where x is positive (hereinafter also referred to the negative frequency influence).
FIG. 13 shows a relation between the measurements obtained from the profile P(x) when the distance dk to the object Mk is at a short distance level (˜10 m) and the actual distance to the object Mk. The relation shown in the figure is obtained under the measurement conditions that central frequency f0 of transmission frequency f is 24.15 GHz and the transmission frequency bandwidth fw is 75 MHz, for the traveling wave D emitted from transmission unit 70.
Referring to FIG. 13, the measurement error generated between the measurement obtained from the conventional distance measuring apparatus and the actual distance dk to the object Mk is larger as the distance to the object Mk is smaller. Specifically, where the distance to the object of measurement Mk is 4 m or longer, the measured value accurately matches the actual distance to the object Mk, while the measurement error abruptly increases where the distance becomes shorter than 4 m. In a region where the distance to the object of measurement Mk is 2 m or shorter, the measurement error comes to be as large as about 1000 mm, and the measurement accuracy is degraded significantly. This results from distortion of profile P(x) at a short distance, suggesting that 2 m is the limit of measurable distance.
This will be described in greater detail. When the distance dk=2 m, the period of reception power signal p(f, 0) is c/(2×2)75 MHz, and therefore, the transmission frequency bandwidth fw=75 MHz exactly corresponds to the bandwidth of one period of the reception power signal p(f, x). Therefore, a longer distance dk, which leads to a shorter period, provides an accurate measurement. Accordingly, the minimum detectable distance dmin can be given by the following equation.dmin=c/2fW  Equation (4)
When the conventional distance measuring device is used as the specified low power radio station, the available frequency bandwidth is limited by the laws like the Radio Law. For instance, according to the section of “Sensors for Detecting Moving Object” of the Japanese Radio Law, when the frequency band of 24.15 GHz is used, the tolerable value of the occupied bandwidth is defined to be 76 MHz. Therefore, as in the example of FIG. 13, the result of measurement involves significant error in measuring a position at a short distance of 2 m or shorter.
The measurement error described above is particular to short distance where the reception power signal p(f, x) comes to be equal to or smaller than one periodic component. Even at a distance involving periodicity not smaller than one periodic component (middle to far distance), there might be a measurement error of a few millimeters (mm), as will be described in the following.
FIG. 14 shows a relation between the measurements obtained from the profile P(x) when the distance dk to the object Mk is at a long distance level (˜20 m) and the actual distance to the object Mk. The relation shown in the figure is obtained under the measurement conditions that central frequency f0 of transmission frequency f is 24.15 GHz and the transmission frequency bandwidth fw is 75 MHz, for the traveling wave D emitted from transmission unit 70.
Referring to FIG. 14, when the position of the object Mk is changed from the distance dk=10 m to dk=20 m at a long distance level, there is an error of about ±2 mm at the largest in the result of measurement. One possible cause of the error is that window length of a window function for Fourier transform is not an integer multiple of the waveform of reception power signal p(f, 0).
Further, at a middle to long distance, measurement error occurs when the position of the object Mk changes slightly, even though the reception power signal p(f, 0) has periodicity.
FIG. 15 shows measurement error when the position of the object Mk is changed slightly within the range of ±10 mm, using the position at a distance dk=10 m as a reference.
As is apparent from FIG. 15, even when the object Mk is positioned at a distance of dk=10 m, which is far from the distance measuring device and where the reception power signal p(f, 0) has sufficient periodicity, the result of measurement involves an error of about ±5 cm.
Here, the following approaches may be taken to reduce the measurement error. As a first approach, at the time of Fourier transform of the reception power signal p(f, 0), a signal range including at least one periodic component is extracted from the reception power signal p(f, 0) and subjected to Fourier transform, and this process is repeated over the range of at least half the periodic component. From each data after Fourier transform, a sum of each time domain is calculated.
As a second approach, a reception power signal is obtained by slightly shifting the initial frequency to be transmitted, with the bandwidth fw used for the transmission frequency f being the same, and the obtained reception signal is subjected to Fourier transform. This process is repeated over the range of at least half the periodic component. From each data after Fourier transform, a sum of each time domain is calculated.
FIG. 16 shows the result of processing when the reception power signal p(f, 0) is multi-processed in accordance with these approaches. As can be seen from FIG. 16, the error observed in the result of measurement is improved to about ±1 cm, with the range of displacement being ±10 mm.
Such a multi-processing, however, includes a plurality of Fourier transform processes, and therefore, takes considerably long time. Thus, it is not suitable for an application that requires quick response.
The distance measuring device shown in FIG. 10 further has a problem that the result of measurement has some error when the object of measurement Mk is moving on an axis of measurement (x-axis) at a constant speed.
More specifically, when the object of measurement Mk is moving, Doppler shift occurs in the reception power signal p(f, 0) of the standing wave S detected at detecting unit 80, in which the reception frequency is shifted from the transmission frequency f by the frequency in proportion to the time-change of the propagation medium. The amount of shifting here acts to decrease the reception frequency when the object Mk is coming closer, and acts to increase the reception frequency when the object Mk is moving away.
By way of example, assume that the object of measurement Mk positioned at a prescribed distance dk=10 m is moving at a constant speed. Further, assume that the conventional distance measuring device performs, at transmission unit 70, upward frequency sweep in which the transmission frequency f is increased within the used bandwidth during sweeping and downward frequency sweep in which the transmission frequency f is decreased within the used bandwidth during sweeping.
At this time, dependent on the direction of movement of the object of measurement Mk and on the direction of sweeping transmission frequency f, the following phenomenon occurs periodically in the reception power signal p(f, 0). Specifically, during upward sweep, the periodicity becomes longer when the object Mk is coming closer, and the periodicity becomes shorter when the object Mk is going away. During downward sweep, the periodicity becomes shorter when the object Mk is coming closer, and the periodicity becomes longer when the object Mk is going away.
When the periodicity becomes longer, periodicity of at least one period is lost in the reception power signal p(f, 0) in the transmission frequency bandwidth fw regardless of the sweeping direction, so that the measurement error caused by the negative frequency influence mentioned above increases.
Here, the measurement error is influenced by the Doppler shift described above, and hence, the error from the actual position (10 m) of the object of measurement Mk increases as the speed of movement of the object Mk increases, both in the upward and downward sweeps.
As an approach for reducing such measurement error, a method may be adopted in which a result of measurement (hereinafter also referred to as first position information) obtained by Fourier transform of the reception power signal p(f, 0) obtained by upward sweep of the transmission frequency f and a result of measurement (hereinafter also referred to as second position information) obtained by Fourier transform of the reception power signal p(f, 0) obtained by downward sweep of the transmission frequency f are obtained, and a correction process is performed by finding an average of the first and second position information, so that the position of the moving object is detected.
FIG. 17 shows the result of correction in accordance with this method. Referring to FIG. 17, when the sweep time is 10 msec., the measurement error is kept 0 m in the range where the speed of movement is at most about ±2 m/s. It can be seen, however, that the measurement error increases when the speed of movement increases, exceeding this range. This comes from the negative frequency influence mentioned above. Specifically, in the conventional distance measuring device, only the local maximum of the region where x is positive is uniformly extracted in the waveform of profile magnitude |P(x)|, and therefore, it is difficult to accurately detect the object of measurement Mk that is in the region where x is negative and is moving away at high speed.
Though not shown, by making shorter the sweep time, it is possible to enlarge the range of speed of movement in which the accurate position of the object of measurement Mk can be obtained.
In this method, however, the range of speed of movement of the object of measurement Mk that allows correction depends of the sweep time of transmission frequency f. Accordingly, for the object Mk moving at high speed, the sweep time must be made even shorter, that is, the speed of sweep must be increased. For this purpose, a new, stable, high-speed-variable oscillator is necessary.